1. Field of the Invention
The present invention relates generally to tissue treatment systems and in particular to methods of determining viability of cells in vivo.
2. Description of Related Art
Wounds, however created, require aggressive debridement in order to satisfactorily remove any foreign or infectious material. Other detritus and necrotic tissue must also be removed in order to insure successful progression along the wound healing pathway. Early identification of viable vs. non-viable tissue would be useful to both the surgeon and the patient. Not only would it keep the patient from enduring additional, painful surgery, but it may also help with treatment outcomes (i.e. reducing the severity of cases, preventing removal of viable tissue and enhancing functionality). Also, identification of non-viable tissue would provide a higher level of confidence that the correct tissue was being removed and the right amount of tissue was being removed. Proper identification of tissue as non-viable would mean that one was less likely to leave visually marginal tissue.
In the case of burns, large traumatic wounds, and some chronic wounds, there exist multiple zones of tissue damage. For example, in traumatic muscle injuries, injury may cause irreversible atrophy of the muscle. In such cases, free muscle transfers may be performed to restore some function. In certain cases, it may take up to a year to determine that the muscle is non-viable and surgery is required. It is known that early treatment (muscle transfer) may lead to better outcomes, and that delay in treatment limits the reconstruction options (Barrie et al., 2004). On the other hand, the more healthy tissue that remains following debridement, the better the outcome. Identification of viable tissues would prevent inadvertent removal of viable tissue.
The differential levels of tissue damage are perhaps most classically described as the “Jackson zones” identified in burns (Jackson, 1953). The most severely and irreversibly damaged area is known as the zone of coagulation due to the destruction of the local proteins. This area is clearly unsalvageable. It is necrotic, often blackened and charred, and must be removed. The most peripheral and least damaged area is known as the zone of hyperemia. Tissue in this region generally completely recovers from the trauma unless it becomes infected or suffers prolonged hypoperfusion.
Tissue in the intermediate zone of stasis has been injured and is potentially salvageable. This is known as the zone of stasis. In mid to deep burn injuries, these wounds can not be salvaged or convert to the zone of necrosis. As reported by Molnar et al. (2005), “This is characterized by increased vascular permeability, edema, and progressive blood viscosity, leading to thrombosis and additional tissue death. It is this zone of stasis that represents the deep second-degree burn that is clearly viable tissue when the patient arrives but subsequently goes on to die and requires excision and grafting much in the manner of a third-degree or full-thickness burn.” When this occurs, wound healing is impeded, and the patient may have to go back in for additional, painful, debridement.
As stated above, the viability of the tissue is dependent upon the ability of the cells in this area to recover from the physiological insults arising from the burns. If the cells are able to receive adequate perfusion and nutrients in a timely fashion, the tissue may survive. If on the other hand, this does not occur, as edema increases, perfusion decreases, tissue oxygenation decreases and the injury progresses resulting in cell and tissue death over the 48-72 hours post-injury. Similar zones of injury, albeit not as visually striking, occur in traumatic wounds, as well as chronic wounds such as decubitus, to various tissues.
The Faustian quandary for the surgeon is whether to (1) conservatively debride, allowing some of the marginal tissue to stay in place and weighing the balance between whether the tissue will respond to the resuscitation efforts or whether the tissue will succumb, become necrotic, provide a nidus for infection and have to be removed at a subsequent procedure or (2) aggressively debride well beyond the margin of the clearly injured tissue, potentially removing viable or recoverable tissue, and by taking this wide swath of tissue severally limiting options for future reconstructive options and hence, functionality.
Currently, debridement and tissue removal in traumatic injuries generally depends upon the surgeon's knowledge of viable tissue morphology. However, in many instances this is not 100% accurate. Areas may look questionable, and it is not until later that it is determined that the tissue is nonviable. At this point, another trip back to the operating room, and another painful debridement is warranted. Conversely, traumatic injuries may be treated by aggressive tissue removal. Tissue which may be viable or recoverable may be removed, limiting future functionality.
Thus, in wound healing, repeated surgical debridement procedures can be required. If senescent or non-viable cells are left at the wound edge, the wound may fail to progress towards healing. A need therefore exists for a method that provides clear identification of the areas of the tissue that have exhibited clear markers of having succumbed to the injury. The surgeon will then know which areas should be excised at the time of the debridement and which should be allowed to remain so that the tissue can recover from the insult and serve as a platform for any reconstruction that may be required in the future.
Efforts to identify senescent tissue in vivo include those of US 2007/0197895 A1, describing an instrument that emits and receives acoustic signals. Also, WO07/130,423A2, describes methods for identifying a margin for debridement by obtaining tissue samples from the tissue site and evaluating expression profiles of the samples, where tissue from within a wound has a different gene expression profile from tissue adjacent to a wound.
There is a need for additional methods that allow a precise and unambiguous continuous identification of viable and nonviable cells in a timely manner, e.g., on the edge of wounds, to determine where debridement should take place. The present application addresses that need.